Drug addiction is a chronic disease characterized by high relapse rates, even following extended abstinence from drug use. Altered glutamatergic plasticity at nucleus accumbens (NAc) synapses after chronic cocaine is identified as a key feature underlying relapse vulnerability. Among the important neuroadaptations that have been identified in NAc are enhancement of prefrontal cortex (PFC) release probability, changes in glutamate receptor composition, expression and currents, and alterations in dendritic spine morphology. However, the underlying mechanisms of drug-induced changes in NAc synaptic plasticity are not fully known. The voltage gated Ca2+ channel (VGCC) auxiliary subunit 2-1 is upregulated by non-contingent administration of multiple drugs of abuse. The 2-1 subunit couples to various subtypes of VGCCs which control neurotransmitter release, influence neuronal excitability, and modify gene transcription. In addition, astrocyte-derived thrombospondin (TSP) proteins bind to 2-1 facilitating the formation of excitatory synapses through a Ca2+ channel-independent mechanism. Gabapentin (GBP), an 2-1 ligand, disrupts the interaction between TSP and 2-1 and inhibits synapse formation. My preliminary data show that 2-1 and TSP-1 are increased after extinction from cocaine self-administration. Moreover, I find that intra-NAc delivery of GBP decreases cocaine-induced reinstatement of drug-seeking. The current proposal is aimed at further clarifying GBP's actions at the 2-1 subunit and determining the mechanism of action of GBP on cocaine-induced drug- seeking. I hypothesize that upregulated 2-1 promotes relapse to cocaine-seeking after self-administration, and conversely, inhibiting 2-1 reduces cocaine-induced reinstatement. I further hypothesize that GBP's effects are due to its actions at presynaptic 2-1 subunits to inhibit neurotransmitter release induced by VGCC activation OR disrupting the scaffolding of TSP proteins thereby interfering with synaptic remodeling. Specific Aim 1: I will knockdown 2-1 with anti-sense vivo-morpholino to validate the role of 2-1 in cocaine reinstatement. Specific Aim 2: Using in vivo microdialysis, I will evaluate my hypothesis that GBP's actions involve inhibition of neurotransmitter release by measuring levels of glutamate and dopamine during cocaine- induced reinstatement with reverse-dialysis of GBP. Specific Aim 3: I will determine the effects of recombinant TSP on cocaine reinstatement and identify whether drug-induced dendritic spine changes are modified by GBP or recombinant TSP treatment using quantitative dendritic spine morphology measurements. Results from these behavioral and biochemical studies will shed light on a shared neurobiological mechanism contributing to drug relapse and clarify the mechanism of action of GBP as an anti-addiction agent.